Spacing - most forks use a 20mm thru-axle (QR20), quick release/QR (9mm), or now, a 15mm hub. It is important to know which hub your fork needs as standard quick release hubs cannot be converted to use with a 20mm compatible fork. However, there are reducers for some hubs that can go from 20mm to standard QR. Rear hubs typically come in two sizes. The first is 150x12 and the second is 135x10. However, some bikes do use a 165mm hub and 135x12. It is important to know which rear hub is required on your bike so it will be compatible.